Experimental study of electrode effects of resistance type electrodes for monitoring wood drying process above fibre saturation point / Elektroodefektide uurimine puidu kuivatamisel niiskussisaldustel üle kiu küllastuspunkti

Abstract

Due to practical need to improve the accuracy of measurement of wood electrical resistance via creating multivariate calibration models the study of electrode effects was carried out. The experiments were made in DC mode and five types of electrode effects were measured: electrical resistance of the galvanic contact between the measuring electrode and wood, corrosion of measuring electrodes in wet wood, polarization and depolarization of the double layer forming on the surface of the measuring electrodes, residual polarization voltage between measuring electrodes and effect of repeated use of measuring electrodes. For multi-channel measurement mode effect of neighbouring electrodes was measured. Behind each of these factors is a complex transfer mechanism of free and bounded charge carriers from wood to measuring electrodes. Among the factors, polarization and depolarization have an immediate effect (duration of the processes in seconds) while resistance of electrode/wood contact, corrosion of electrodes and residual polarization have a slow effect (duration of processes in hours). For pine sapwood, coefficients of the Stamm formula were found in moisture contents (MC) above the fibre saturation point. It was determined that wood polarization and depolarization indicators are dependent on wood moisture content as is the case with wood electrical resistance. Compared to wood electrical resistance, wood polarization proved approximately four times and depolarization approximately eight times less sensitive to moisture content variation.

Experimental study of resistance type wood moisture sensors for monitoring wood drying process above fibre saturation point/ Takistus-tüüpi puidu niiskuse andurite eksperimentaalne uurimine puidu kuivatamise monitooringul niiskussisaldustel üle kiu küllastuspunkti

“…Details on model repeatability conditions (i.e. validation) have been analysed in the research by Tamme et al (2013).…”

“…The effect of repeated relocation of insulated pin electrodes in new measuring areas when measuring wood electrical resistance has been quantitatively analysed with the S-value (Rozema, 2010; and the statistical modelling method (Tamme et al, 2013). The last study quantitatively evaluated the progressive decrease in the measuring accuracy of some widely used wood moisture meters upon an increasing wood average moisture content from 30% up to 100% (above FSP).…”

“…However, the method is defi cient due to the fact that above fi bre saturation point (FSP) measuring accuracy is insuffi cient and starts decreasing correspondingly to the increasing wood moisture content (Edwards, 1974;James, 1993;Vermaas, 2002). The decrease in measuring accuracy can also be described quantitatively, with the 2S-value (Rozema, 2010; or by statistical modelling (Tamme et al, 2013). Insuffi cient measuring accuracy above FSP may also reduce the accuracy of effective diffusion coeffi cient monitoring in the process of wood drying (Tamme et al, 2010(Tamme et al, , 2011.…”

Novel Wood Resistance Measurement Method Reducing the Initial Transient Instabilities Arising in DC Methods Due to Polarization Effects

“…For a certain electrode position separated an L distance, the wood electrical resistance between the electrodes is R = ρL/A, where A is the current area transversal section. Thus, resistance R depends on the geometry of the wood piece, the electrode position, the material of the electrodes [13,15,16], the insertion depth of the electrodes, the wood species and its associated density, heartwood and sapwood [17], wood temperature and its MC [3]. Most of these parameters affecting the measured value can be controlled or adjusted by using standard measurement protocols as the EN 13183-2:2002 European Norm [18] for sawn timber or EN 16682:2018 [19] describing the limits and conditions by which the resistive measurement method can be applied to wood and conservation of cultural heritage.…”

“…Finally, Section 5 deals with conclusions. [15] its corrosion, wood polarization and depolarization effects, etc. on the wood resistance measured.…”